1. Field of the Invention
This invention relates to logging wells during drilling, and more particularly to the wireless telemetry of data related to downhole conditions.
2. The Prior Art
It has long been the practice to log wells by sensing various downhole conditions within a well and transmitting the acquired data to the surface through a wireline or cable-type equipment. To conduct such logging operations, drilling is stopped, and the drill string is removed from the well. Since it is costly and time-consuming to remove the drill string, the advantages of logging-while-drilling, or at least without removing the drill string from the well bore, have long been recognized. However, the lack of an acceptable telemetering system has been a major obstacle to successful logging-while-drilling.
Various systems have been suggested for logging-while-drilling. For example, it has been proposed to transmit data to the surface electrically through wires. Such methods have been impractical because of the need to provide the drill string sections with a special insulated conductor and appropriate connections for the conductor at the drill string joints. If a steering tool is used for directional drilling, and is controlled by wires from the surface, the wires and tool must be withdrawn from the well before continuing drilling in the rotary mode. Other proposed techniques include the transmission of acoustical signals through the drill string. Examples of such telemetering systems are shown in U.S. Pat. Nos. 3,015,801 to Kalbfell and 3,205,477 to Richards. In those systems, an acoustical signal is sent up the drill string and frequency modulated in accordance with a sensed downhole condition.
Wireless systems have also been proposed using low-frequency electromagnetic radiation through the drill string, borehole casing, and the earth's lithosphere to the surface of the earth.
Other telemetering procedures proposed for logging-while-drilling use the drilling fluid within the well as a transmission medium. U.S. Pat. Nos. 2,759,143 and 2,925,251 to Arps and 3,958,217 to Spinnler disclose systems in which the flow of drilling fluid through the drill string is periodically restricted to send positive pressure pulses up the column of drilling fluid to indicate a downhole condition. U.S. Pat. Nos. 2,887,298 to Hampton and 4,078,620 to Westlake et al disclose systems which periodically vent drilling fluid from the drill string interior to the annular space between the drill string and the well borehole to send negative pressure pulses to the surface in a coded sequence corresponding to a sensed downhole condition. A similar system is described in U.K. Patent Publication No. 2,009,473 A (Scherbatskoy).
A general problem with using pressure-pulsing equipment in a drill string to send information through the drilling fluid is that the pulse generators to date have been bulky and, therefore, impose a wasteful pressure drop in the drilling fluid flowing through the drill string. Moreover, the previous pulse generators require a relatively large amount of electrical power, which means short operating time if batteries are used. The previous pulse generators also tend to plug when the drilling fluid includes lost circulation material (LCM), and are subject to excessive wear, resulting in short service life and frequent failure under operating conditions.
In addition, some of the prior art pulse generators require specially built drill collars in the drill string to receive the generators and cannot reliably be positioned in the lower end of the drill string without removing the drill string from the well bore.
U.S. Pat. No. 4,550,392 to Mumby discloses an improved pressure pulse generator which overcomes many of the disadvantages of the prior art. However, we have found that the pressure pulse generator shown in that patent is sometimes subjected to excessive vibration, which shortens its service life.
The present invention provides an improved pressure pulse generator less subject to vibration or problems with LCM and, therefore, with a longer and more reliable service life.
Another advantage of the present invention is that when it is in operating position in the drill string, it offers a relatively low resistance to flow of drilling fluid, and is more tolerant of LCM, which is sometimes added to the drilling fluid for well control.
The pulse generator of the invention can be used to transmit data which measures many different downhole conditions, such as electrical resistivity, radioactivity, temperature, drilling fluid flow rate, weight-on-bit, torque, and the like. It is also well suited for directional survey work, i.e., determining the inclination and azimuth of a borehole. Such information is important for ascertaining that the well is being accurately drilled to a selected downhole position.